1. Field of the Invention
The present invention relates to turboshaft engines, and in particular to turbojet aero-engines of the bypass type.
Such a turboshaft engine conventionally comprises, proceeding from upstream to downstream in the direction of primary flow through the engine, an axial compressor supplying compressed air to a combustion chamber in which the air is mixed with pressurized fuel which is burnt to power an axial turbine which drives the compressor, the gases discharged from the turbine providing the thrust used for the propulsion of the engine.
Such turbojet engines of purely axial configuration take in at their front intake not only air required for operation but also, depending on climatic conditions, sand and/or water which can represent rather an impediment to their operation. This is particularly the case when an aircraft flies into a storm or through a large volume cloud such as a cumulus or cumulo-nimbus. Substantial quantities of water in the form of rain or hail can then enter the compressor. If the engine is at full throttle the water is vaporized and, even if it reaches the combustion chamber, the water is in the form of steam which is sufficiently hot and atomized not to extinguish the combustion chamber, which is supplied by a substantial flow of fuel.
Such is not the case, however, when an aircraft is descending, e.g. in the approach stage prior to landing. In this case, the engine is running slowly and the compression ratio of the compressor is relatively low, and water in the liquid or solid state, e.g. in the form of large drops or ice particles or even as "sheets of water", may reach the combustion chamber and extinguish the combustion of one of the burners, even all of them, or lead to the engine cutting out, the amount of fuel supplied being relatively low. However, this can happen only in exceptional cases where the weather conditions exceed the conditions taken into account when designing the engine.
In the majority of cases the sparking plugs may also be wetted and become temporarily useless. When extinguishing of the combustion chamber occurs, if the pilot is unable to leave the critical rain area, the engine(s) could then be fully stopped, with all the risks this would involve.
To prevent such situations it is imperative to get rid of as much as possible the water ingested before it reaches the combustion chamber, or, if it does reach the chamber, to make sure that it cannot put the burners out.
2. Summary of the Prior Art
Several means in the region of the combustion chamber may be used to do this, including water deflectors compelling the air to follow complex paths before entering the burners, paths which the water cannot follow because of its greater inertia.
Another way of preventing water from reaching the combustion chamber consists of exhausting the excess water at the exit of the low pressure compressor before the air charged with liquid or solid water enters the high pressure compressor.
The present invention is concerned with a way of achieving this, based on the existence of blow-off valves which are usually provided between the compressors of twin-flow and twin body turbojet engines. A peculiarity of these valves is that they are open when the engine is idling.
As shown in FIG. 1, which illustrates this arrangement in a conventional engine, these blow-off valves 1 are traditionally arranged in the duct 2 separating the low pressure compressor 3 from the high pressure compressor 4 and, when they are open, they create a discharge route 5 for part of the air flow compressed by the low pressure compressor towards the secondary flow path 6. This arrangement is operative when, in order to prevent the risk of possible surging of the low pressure compressor, it is necessary to allow a flow of air to pass through it greater than that which could be absorbed further downstream by the high pressure compressor. The excess air flow is therefore exhausted to the secondary flow path at a position between the low pressure compressor and the high pressure compressor.
It is found that this situation occurs when the turbojet engine is running at low speeds, and the opening behaviour of the blow-off valves is represented by curve A in FIG. 2. When the engine is idling and until it achieves a rotational speed close to one third of its speed at full throttle, the blow-off valves are fully open, their angle of opening reaching 45.degree. or more. Their opening then reduces as the engine accelerates until the closed position is reached when moving from cruising speed to full throttle.
It is precisely the conditions in which these valves are open that the most dangerous water ingestion conditions are encountered. However the known valves, although they discharge air charged with water under these conditions, cannot bail out the water to a maximum extent as they have no part projecting into the flow path.
It is an object of the invention, therefore, to adapt these valves in a simple manner to enable them to collect more efficiently the particles of solid or liquid water centrifuged against the outer wall of the flow path, the centrifuging originating from the rotation effect imparted by the compressor Positioned upstream as well from the swan neck shape of the duct which contains the valve.